1. Field of the Invention
The present invention relates to a motor drive system for a railway vehicle.
2. Description of Related Art
Examples of motor drive systems for railway vehicles are disclosed in Japanese Unexamined Patent Application Publication No. 2000-308388, Japanese Patent Publication No. 3594100, and the like. FIG. 1 is a circuit diagram showing a motor drive system 100 for a railway vehicle according to a related art. FIG. 2 shows an inverter 110 of the motor drive system 100, wheels 120, a motor 130, and the like arranged in the railway vehicle of FIG. 1.
To make the motor 130 free of maintenance, the motor drive system 100 of the related art employs, as the motor 130, an AC motor such as an induction motor or a permanent-magnet synchronous motor. The motor 130 is connected to and driven by the inverter 110. The inverter 110 is installed under a vehicle floor 140 and is connected to the motor 130 through electric wiring 150 laid along the vehicle floor 140. In these years, inverters widely employ high-performance semiconductor switching elements such as IGBTs (Insulated Gate Bipolar Transistors) that withstand high voltages, e.g. 3300 V and achieve high-speed switching.
To improve the acceleration of a railway vehicle, a large current for driving a high-output motor is needed. To pass such a large current, wiring laid between the motor and the inverter will become heavier. High-speed switching of an inverter produces current harmonics, which are passed from the inverter to a motor through long wiring. The current harmonics passed through long wiring cause electromagnetic noise that may cause malfunctions of railway signals. It is preferable, therefore, to minimize the length of the wiring between the inverter and the motor. The inverter, however, is generally bulky, and therefore, there are limits on arranging the inverter as close to the motor as possible to shorten the wiring.
Semiconductor switching elements of high withstand voltage used for railway inverters are special-purpose products that are costly to manufacture. There is, therefore, a need of a motor drive system for a railway vehicle that is manufacturable at low cost to realize economical railway transportation.